1. Field of the Invention
The present invention generally relates to a power control monitoring system for an underwater cable communications system, and more particularly to a power control monitoring system for monitoring a power feed system in an underwater cable communications system in which more than two cable landing stations are connected to each other via an underwater cable branching unit.
2. Description of the Prior Art
An underwater cable communications system, such as an optical underwater cable communications system, has underwater repeaters located at predetermined intervals. It is necessary to feed power to the underwater repeaters. Cable landing stations have power feed equipment (PFE) which feed power to the underwater repeaters through power feed cables. The power of feed cables are provided separately from optical cables for communications.
FIGS. 1A and 1B are diagrams illustrating a procedure for feeding power to repeaters REP provided in a cable connecting two cable landing stations A and B. As shown in FIG. 1A, the cable landing stations A and B communicate with each other via a public or private communications channel provided by a satellite or an existing underwater cable. Power feed equipment provided in the station A sets an output current to a predetermined level, and thereafter a power feed equipment provided in the station A sets an output current to a predetermined level. Hence, as shown in FIG. 1B, a current flows in the cable from the station A to the station B, and the repeaters REP provided in the cable are supplied with power. The power feed shown in FIG. 1B is called a two-end power feed.
FIGS. 2A, 2B and 2C are diagrams illustrating a power feeding procedure for an underwater cable communications system including three stations A, B and C. Normally, as shown in FIG. 2A, a cable branching unit BU provided in the sea connects the three stations A, B and C to each other by means of power feed cables. When a fault has occurred in the system, power feed paths are switched in the cable branching unit BU in order to cope with the fault. The branching unit BU has a high breakdown voltage relay for switching the power feed paths. The relay in the cable branching unit BU can be driven by the cable landing stations by controlling a current or voltage output by the power feed equipment provided in the cable landing stations.
If the potential of the cable branching unit BU is high with respect to the ground potential at the time of switching the relay, a hot-switching phenomenon will occur. In this case, a surge voltage is developed in the power feed cable and a large current flows in the relay. Hence the relay may be damaged. With this in mind, the cable branching unit BU is set to the ground potential before switching in order to prevent occurrence of the surge voltage. When power feed paths are established or switched, it is necessary to control the power feed current or voltage output by the power feed equipment in the landing stations so that the branch unit BU is set to the ground potential. Normally, a procedure for setting or switching the power feed paths BU consists of several power feed steps.
In order to perform the above procedure for the underwater cable communications system in which three stations are connected to each other via the cable branching unit, one of the three stations functions as a power integration coordinator (PIC), and the other two stations function as power safety officers PSO. The power integration coordinator directs power up/down of the overall system, and the two power safety officers work under the control of the power integration coordinator.
Assuming now that the station A functions as the power integration coordinator, as shown in FIG. 2A the station A communicates with the station B (or C) by a communications means, as described previously with reference to FIG. 1A. The stations A and B alternately perform a current/voltage control step several times (six times, for example) in order to disconnect, at the cable branching unit BU, the underwater cable connected to the station C from the power feed path between the stations A and B, to ground the disconnected cable to the sea and to establish a power feed path between the stations A and B. A current flowing in the power feed path between the stations A and B is increased to a predetermined level, the communication between the stations A and B is terminated, and the station A starts to communicate with the station C to inform the station of power up, as shown in FIG. 2B. Then, the station C starts power feed between the station C and the cable branching unit BU, as shown in FIG. 2B.
FIG. 3A shows an underwater cable communications system having four cable landing stations A, B, C and D. The power feed procedure described with reference to FIGS. 2A, 2B and 2C can be applied to the system shown in FIG. 3A as follows. First, as shown in FIG. 3A, the station A, functioning as the power integration coordinator, communicates with the station B. Next, the stations A and B alternately perform the current/voltage control step several times (eight times, for example) in order to start the power feed between the stations A and B via cable branching units BU1 and BU2. Then, the station A terminates communication with the station B. Then, as shown in FIG. 3B the station A communicates with the station C and gives the station C an instruction to start a single-end power feed. The station C starts the single-end power feed between the station C and the ground at the cable branching unit BU1 in response to the instruction from the station A. Thereafter, the station A terminates communication with the station C, and calls the station D to instruct the station D to start a single-end power feed, as shown in FIG. 3C. In response to the instruction from the station A, the station D starts the single-end power feed between the station D and the ground at the cable branching unit BU2, as shown in FIG. 3D.
FIG. 4A shows an underwater cable communications system having four landing points, and FIGS. 4B through 4F illustrate the power feed steps for the underwater cable communications system shown in FIG. 4A. Referring to FIG. 4A, the station A operates in a constant-voltage (CV) regulation mode in which the power feed voltage is controlled so that the underwater cable branching units BU1 and BU2 operate in the grounded state. The station B operates in a constant-current (CC) regulation mode in which a current flowing in the power feed path is adjusted. Relays provided in the cable branching units BU1 and BU2 and used to switch the power feed paths can be activated by operating currents i1 and i2, respectively, where it is greater than i2. Symbols in parentheses denote voltages necessary to flow corresponding currents. When currents equal to or greater than the operating currents i1 and i2 respectively flow in the relays in the cable branching units BU1 and BU2, the relays are activated.
In step 1 shown in FIG. 4B, the station B operating in the constant-current mode flows the operating current i2 in the power feed path between the stations A and B. In step 2 shown in FIG. 4C, the station A operating in the constant-voltage mode calculates a power feed voltage to be applied to the power feed path between the station A and the cable branching unit BU2 in a state in which the operating current i2 activating the relay in the cable branching unit BU2 flows therein. Then, the station A applies the calculated voltage to the cable extending therefrom.
In step 3 shown in FIG. 4D, the station B increases the power feed current to a magnitude between that of the operating current i1 and that of the operating current i2. In the process of increasing the power feed current, the power feed current becomes equal to the operating current i2. At this time, the cable branching unit BU2 operates in the grounded state and the relay provided therein is activated. Hence, the power feed path connected to the station D is disconnected from the power feed path between the stations A and B, and is grounded to the sea.
In step 4 shown in FIG. 4E, the station A calculates a power feed voltage to be applied to the power feed path between the station A and the cable branching unit BU1 in a state in which the operating current i1 activating the relay in the cable branching unit BU1 flows therein. Then, the station A applies the calculated voltage to the cable extending therefrom.
In step 5 shown in FIG. 4F, the station B increases the power feed current to the normal operating current. When the power feed current becomes equal to the operating current i1 in the process of increasing the power feed current, the cable branching unit BU1 starts to operate in the grounded state and disconnects the power feed path extending to the station C from the power feed path between the stations A and B. The disconnected power feed path is grounded to the sea. In step 6 (not shown), the station A increases the power feed voltage to the normal operating voltage.
A procedure for power down can be performed in a manner similar to the above-mentioned manner of power up while two of the stations sequentially communicate with each other.
However, the above-mentioned prior art has the following disadvantages. First, it is necessary for the station functioning as the power integration coordinator to call the power safety officers many times until power up/down is completed. Hence, it takes a longer time to complete power up/down as the number of stations increases.
Second, the power safety officer stations are maintained in a standby state until they receive instructions from the power integration coordinator station, and do not have any information on the status of the system. Hence, there is a possibility that an erroneous operation may be performed and the power up/down operation may fail.
Third, a larger number of power feed steps is needed as the number of stations increases. Hence, the procedure becomes complicated, and becomes difficult to find erroneous operation performed at other stations.